An electric assist apparatus, which electrically generates torque, has been proposed as a mechanism that assists a steering operation of a vehicle in place of a hydraulic assist apparatus, which hydraulically generates torque. The electric assist apparatus provides assist, differently from the hydraulic assist apparatus, only when a driver of the vehicle performs a steering operation.
A brushless motor that is rotationally driven when, for instance, a three-phase alternating current is applied to it is used as a motive power source for the electric assist apparatus. When such a brushless motor is used, coil currents differing in phase are supplied to multi-phase (e.g., three-phase) coils. Therefore, AC outputs differing in phase need to be produced from a DC output having a predetermined voltage (e.g., 12 V). Consequently, it is necessary to use an electronic circuit for selecting a coil current. The electronic circuit includes, for instance, semiconductor modules that provide a switching function, and a microcomputer that provides overall control. It has been proposed that the electronic circuit be positioned near the motor. The semiconductor modules described, for instance, in patent documents 1 and 2 are disposed in the axial direction of the motor. The semiconductor modules described, for instance, in patent document 3 are disposed around a stator that is a part of the motor.    Patent document 1: JP10-234158A    Patent document 2: JP10-322973A    Patent document 3: JP2004-159454A
The electric assist apparatus uses a relatively large motor in order to provide sufficient torque. Thus, the semiconductor modules are large-sized. Further, the electronic circuit generally includes a large-sized capacitor (e.g., aluminum electrolytic capacitor) in order to prevent a semiconductor chip from being damaged by a switching-induced surge voltage.
In recent years, however, various apparatuses are mounted in a vehicle in addition to the electric assist apparatus. Therefore, it is now important that the space necessary for installing various apparatuses be secured. It is thus increasingly demanded that the motor for the electric assist apparatus be reduced in size.
In this respect, the motor disclosed in patent document 2 is large in axial physical size because it includes a cooling fan.
The motor disclosed in patent document 3 is small in axial physical size because the semiconductor modules are disposed around the stator. However, this motor is large in radial physical size. In addition, the radial physical size is further increased in a situation where a cylindrical smoothing capacitor has to be used (although a flat smoothing capacitor is used for the motor).
In the electric assist apparatus that uses a relatively large motor as described above, the semiconductor modules used for the motor generate a relatively large amount of heat. As such being the case, it is demanded that the motor be reduced in size and improved in heat release performance.
If the semiconductor modules are disposed on the surface of a metal member in the same manner as the semiconductor modules for the motor described, for instance, in patent document 1, the heat release performance is degraded by the release of heat from neighboring semiconductor modules.